In a normal operational situation of a vehicle engine, the difference between the exhaust gas temperature, and the ambient air temperature can reach 800° C. or more. The temperature difference can cause large thermal stresses in the exhaust manifold, in turn resulting in a risk of cracks appearing in the manifold. A known way to solve this problem is to use high strength materials for the manifold, which, apart from being expensive, often have the disadvantage of being difficult to process when manufacturing the manifold.
Therefore, inventors herein have recognized it is important to make the manufacturing of exhaust manifolds for internal combustion engines less complex and cheaper and to reduce thermal stresses of an exhaust manifold for an internal combustion engine.
Accordingly, a method for controlling a diesel engine in a vehicle, the engine having at least one cylinder, a fuel injector for each cylinder, and a throttle valve for controlling the flow of air into each cylinder, the method includes: controlling the throttle valve to assume a not fully open position if an amount of fuel injected into the engine cylinder is equal to or less than a fuel injection threshold value.
While applicable to any kind of internal combustion engine with fuel injection and a throttle, the invention is especially advantageous in connection to engines with compression combustion such as diesel engines. The inventors have discovered that diesel engines subjected to a lot of overrun (motor brake), i.e., fuel injection shut-down while the crankshaft is turning, compared to engines subjected to less overrun, present higher amount of material failures in the exhaust manifold. The inventors have also recognised that this problem is growing with further developments in engine technology since there is, especially in diesel engines, an increasing use in overrun situations of power extraction from the engine.
By controlling the throttle valve, if the flow of fuel injected into the cylinder is equal to or less than a fuel injection threshold value, so as to assume a not fully open position, the flow of relatively cold air through the engine, and through the exhaust manifold, will decrease. This will decrease the thermal stresses of an exhaust manifold of the engine when heated. In turn, this will reduce the need to make the manifolds in expensive high-temperature enduring materials.
The not fully open position of the throttle valve can be a position in which the throttle valve is less than fifty percent open, preferably less than twenty percent open. If during a motor brake condition of the engine the throttle valve is less than fifty percent open, the flow of cold air to the exhaust manifold is effectively reduced. The throttle valve being less than twenty percent open further reduces the flow of cold air to the exhaust manifold, while maintaining a sufficiently high pressure in the inlet manifold of most typical vehicle engines.
The fuel injection threshold value can be a predetermined value and is preferably chosen so that at or below it, no or a negligible torque contribution from the fuel injection is given. Thereby, it can be determined that gas passing through the cylinders of the engine will not be heated, and that the exhaust manifold could be exposed to cold gas. It should be noted that the threshold value could correspond to no fuel injection at all taking place.
Preferably, the step of controlling the throttle valve is at least partly based on the engine speed. This provides a possibility to obtain at each given engine speed a good trade-off between the desire to reduce the flow of cold air to the exhaust manifold, and to maintain a sufficiently high pressure in the inlet manifold.
Preferably, the step of controlling the throttle valve is at least partly based on a position of an exhaust gas recirculation valve of the engine.
Preferably, it is determined whether a clutch for a gearbox of the vehicle is engaged, the step of controlling the throttle valve being dependent on the clutch being engaged. In a vehicle with a manual gearbox, determining whether the clutch is engaged contributes to establishing whether the engine is subjected to a motor brake situation.
It should be noted that the invention is also applicable to engines in vehicles with automatic transmission. For example, if fuel injected is equal to or less than the fuel injection threshold value, the throttle valve can be controlled as described above when the automatic transmission is in the highest gear with a so called lockup function engaged, or when the automatic transmission is in a manual mode.
Preferably, it is determined whether the engine speed is above a predetermined engine speed value, the step of controlling the throttle valve being dependent on the engine speed being above the predetermined engine speed value.
Preferably, the temperature of an exhaust manifold of the engine is determined, and the step of controlling the throttle valve is based on the exhaust manifold temperature. In a preferred embodiment, it is determined whether the temperature of the exhaust manifold is above a predetermined manifold temperature value, and the step of controlling the throttle valve is dependent on the exhaust manifold temperature being above the predetermined manifold temperature value. Thereby, unnecessary throttling, for example, immediately after a cold start of the engine when the exhaust manifold is not yet hot, can be avoided.